Propoxylated PTB coalescing agents for water-borne protective coatings

ABSTRACT

Versatile coalescing agents for water-borne coating applications are disclosed. The coalescents comprise propoxylated propylene glycol tert-butyl ether and one or more organic solvents, particularly glycol ethers. These coalescents efficiently reduce minimum film-forming temperature, and also have a dramatically reduced tendency to swell resins compared with prior art coalescents.

FIELD OF THE INVENTION

This invention relates to the development of coalescing agents for usein water-borne protective coatings. We have found that propoxylatedpropylene glycol tert-butyl ether (propoxylated PTB) and mixtures ofpropoxylated PTB with organic solvents are versatile coalescents for thepreparation of coatings with superior film-forming ability and rapidhardness development.

BACKGROUND OF THE INVENTION

Coalescing agents, or coalescents, are key components in water-borneprotective coatings. Glycol ethers, such as ethylene glycol n-butylether (EB), diethylene glycol n-butyl ether (DB), propylene glycolmonomethyl ether (PM), and dipropylene glycol monomethyl ether (DPM),are commonly used in water-based systems. The glycol ether functions asa coalescent which couples dispersed or emulsified polymer resinparticles with water, and helps to keep them dispersed.

Water-borne systems have become increasingly important for environmentalreasons, and also because water-borne systems now offer comparableperformance to solvent-based systems. Water-borne systems includedispersions and emulsions. Dispersions contain resins such as epoxies,alkyds, polyesters, and acrylics, which can be converted to charged,water-soluble species upon combination with a neutralizing agent.Following application of a water-based dispersion, the water evaporatesmore quickly than most of the coalescent; the coalescent softens theresin, and allows resin particles to fuse together to form a continuousfilm. Emulsions contain resins such as acrylic, styrenebutadiene, andpolyvinyl acetate. These resins form latex spheres that are emulsifiedin water with the aid of a surfactant. Upon application, the coalescentevaporates slowly enough from the latex spheres to allow them tocoalesce.

Ethylene glycol-derived ethers (E-series ethers) are commonly used inwater-borne coatings. Unfortunately, many E-series ethers are toxic, andworker exposure to these chemicals is now regulated. Glycol ethersderived from propylene glycol (P-series ethers), including propyleneglycol tert-butyl ether (PTB), offer a low-toxicity alternative toformulators, as described by R. Heckman ("Using P-Series Glycol Ethersin Water-Reducible Coatings," Modern Paint and Coatings, June, 1986, pp.36-42). Propoxylated PTB is expected to have toxicity characteristicssimilar to those of PTB.

Ethylene glycol n-butyl ether (EB) is widely used in the coatingsindustry as a coalescing agent. In addition to its acute toxicity, EBhas an undesirable tendency to swell water-borne resins. Unfortunately,the swelling problem is rather unpredictable and varies dramaticallywith changes in pH. As a result, coating compositions containing EB haveviscosities that are highly pH-dependent, especially within pH=8-9, arange commonly employed in formulating water-borne coatings. Coatingcompositions made with EB are therefore difficult to use.

The acute sensitivity of formulation viscosity to changes in pH whenethylene glycol n-butyl ether (EB) is employed as a coalescent is wellillustrated in "RHOPLEX WL Emulsions for Acrylic Lacquers for IndustrialFinishing--Formulator's Guide," (Rohm and Haas Company, December 1987)on pages 9 and 10. As shown in FIG. 3E of this reference, the 24-hourequilibrated viscosity of a "RHOPLEX WL-91" water-based formulation withEB as the coalescent increases dramatically from about 1000 cps atpH=7.8 to greater than 10,000 cps at pH=8.4. Because of this steepdependence of viscosity on pH, formulators using conventionalcoalescents such as EB or EB/DB blends have needed to exercise greatcare in controlling pH; more flexibility would be highly desirable.

To overcome the pH/viscosity sensitivity problem, formulators commonlyincrease the level of solvents (typically volatile organic compounds)used in the formulations. Because of increasingly strict emissionregulations, coating formulators are being forced to lower the VOCcontent (increase the percent solids content) of their coatings. Findingcoalescents that reduce the viscosity/pH sensitivity of resins wouldenable formulators to decrease the level of viscosity-reducing solventsneeded, thereby reducing VOC emissions and formulation cost.

Some glycol ether coalescents, such as propylene glycol tert-butyl ether(PTB), evaporate too rapidly to allow film formation for someformulations. Others evaporate too slowly, resulting in coatings withpoor hardness development profiles.

Another problem in the art is to find suitable coalescents that alsohave the ability to give coatings with satisfactory minimum film-formingtemperatures (MFFT) at low coalescent levels.

It is therefore an object of this invention to provide coalescing agentsthat have utility in water-based protective coatings, safe handlingcompared with E-series glycol ethers, viscosities that are relativelyinsensitive to changes in pH, satisfactory evaporation rates, and goodminimum film-forming characteristics.

SUMMARY OF THE INVENTION

This invention provides versatile coalescing agents for use inwater-based protective coatings. The coalescents are useful forpreparing coatings with good film-forming and hardness developmentcharacteristics. In addition, coating compositions made with thecoalescents of the invention have relatively flat pH-viscosity profiles,which enables formulators to make coatings with reproducibleviscosities.

The coalescents of the invention comprise:

(a) from about 1 to 99 weight percent of one or more propylene oxideadducts of propylene glycol tert-butyl ether having the formula:##STR1## wherein n is an integer from 2 to 6, and A is an oxypropyleneunit; and (b) from about 1 to 99 weight percent of an organic solvent.Preferably, the organic solvent is selected from the group consisting ofalcohols, glycols, glycol ethers, glycol ether esters, ketones, esters,aliphatic and aromatic hydrocarbons, halogenated hydrocarbons, nitriles,amides, and carbonates.

The invention also provides water-borne coating compositions thatinclude the coalescents described above. The compositions comprise awater-borne resin, water, and a coalescent.

DETAILED DESCRIPTION OF THE INVENTION

The propoxylated PTB glycol ether coalescents of the invention may beprepared by any number of methods well known to those skilled in theart. Propylene glycol tert-butyl ether (PTB), a glycol ether known inthe art as a useful cosolvent for water reducible coatings (see ModernPaint and Coatings, June 1988, pp. 36-42), can be reacted with propyleneoxide under basic conditions to give propoxylated PTBs, as illustratedin U.S. Pat. No. 4,888,446. Generally, a distribution of productsresults from propoxylation of the PTB. If one uses, for example, twomoles of propylene oxide (PO) for each mole of PTB, a complex mixture ofproducts typically results, and oligomeric products containing from 0 to6 or 7 A units comprise the bulk of the product mixture. The glycolethers can also be prepared from the reaction of glycols such asdipropylene glycol, tripropylene glycol, etc., with isobutylene underacidic conditions.

The coalescents of the invention include propylene oxide adducts ofpropylene glycol tert-butyl ether having the general formula: ##STR2##wherein n is an integer from 2 to 6, and A is an oxypropylene unithaving the structure: ##STR3## The oxypropylene units may be configuredin any manner; for example, the oxypropylene units may be primarily orexclusively of one variety, such as all ##STR4## units. Alternatively,an essentially random distribution of ##STR5## units may be present,depending on the method of preparation. The glycol ethers may thereforehave either secondary or primary hydroxyl end groups. Mixtures of glycolethers having either primary or secondary hydroxyl end groups may alsobe used.

The coalescents of the invention are produced by combining from about 1to 99 weight percent of a glycol ether as described in the precedingparagraph with from about 1 to 99 weight percent of an organic solvent.Preferably, the organic solvent is selected from the group consisting ofalcohols, glycols, glycol ethers, glycol ether esters, ketones, esters,aliphatic and aromatic hydrocarbons, halogenated hydrocarbons, nitriles,amides, and carbonates. The relative amounts of propoxylated PTB andorganic solvent used will depend on the particular application. Mixturesof organic solvents may be used to optimize specific coating propertiesor achieve satisfactory resin solubility. Preferred organic solvents areE- and P-series glycol ethers, including, but not limited to, propyleneglycol tert-butyl ether, propylene glycol n-butyl ether, propyleneglycol methyl ether, propylene glycol ethyl ether, propylene glycolphenyl ether, ethylene glycol n-butyl ether, diethylene glycol ethylether, and the like, and mixtures thereof. Particularly preferred ispropylene glycol tert-butyl ether.

The coalescing agents of the invention are useful in water-borne latexcoating compositions. These compositions typically comprise: (a) fromabout 20 to 60 parts by weight of a water-borne resin; (b) from about 15to 80 parts by weight of water, and (c) from about 1 to 25 parts byweight of a coalescing agent selected from the group consisting of oneor more propylene oxide adducts of propylene glycol tert-butyl ether andmixtures thereof with one or more organic solvents.

The water-borne resin may be any dispersion or emulsion-type resin knownin the art to be useful for making protective coatings. Examples ofsuitable water-borne resins include, but are not limited to, alkyds,epoxies, polyesters, polyurethanes, acrylics, styrene resins,styrene-acrylic resins, styrenebutadiene resins, polyvinyl acetateresins, and the like, and useful combinations thereof.

The water-borne coating compositions of the invention may includeadditives such as flow-control agents, adhesion promoters, curecatalysts, surfactants, plasticizers, pigments, mar aids, and the like.Such additives and the methods of using them are well known in the art.

Amines or alkanolamines may be added to the coating compositions asneeded to adjust the pH of the compositions. Typically, the pH of mostcoating compositions falls within the range of about 7-10, moretypically about 8-9. Neutralization of water-borne coating systems isoften desirable because charged polymer particles tend to have betterwater dispersibility, and thus neutralization with an amine oralkanolamine can help to stabilize a coating formulation. Examples ofsuitable amines and alkanolamines include, but are not limited to,aqueous ammonium hydroxide, triethylamine, triethanolamine,ethanolamine, N,N-dimethylethanolamine and the like, and mixturesthereof. Particularly preferred is aqueous ammonium hydroxide.

Generally speaking, the coating compositions of the invention areprepared by combining, in any order desired, a water-borne resin, water,and the propoxylated PTB coalescent, together with any desiredadditives, such as cosolvents, plasticizers, surfactants, mar aids, flowaids, etc. The formulations are blended well until a homogeneous productis obtained.

The coating compositions may be applied to a substrate by any of themethods known in the art, such as brushing, spraying, dipping, and thelike. The newly applied coating is typically allowed to dry at ambientor elevated temperature. Oven-drying may be used if desired toaccelerate the rate of drying.

Propoxylated PTB is a versatile coalescent. Properties can often betailored to fit specific applications either by blending with anotherglycol ether such as PTB, or by varying the average number ofoxypropylene units present. Less-volatile coalescents may be easilyprepared by adding more A units to the molecule.

The following examples illustrate the invention. Those skilled in theart will recognize many possible variations that are within the spiritand scope of the claimed invention.

PREPARATION OF MONO-PROPOXYLATED PTB

"ARCOSOLV PTB" propylene glycol tert-butyl ether (500 g) was combinedwith potassium hydroxide pellets (28.6 g) in a three-neck round-bottomflask. The mixture was distilled under nitrogen at atmospheric pressureto remove the PTB/water azeotrope, giving a product residue (383 g)containing 7.5% potassium.

An autoclave reactor was charged with 201 g of the alkoxide produceresidue described above, along with propylene glycol tert-butyl ether(2261 g). The mixture was stirred and heated to 110° C. Propylene oxide(538 g) were added over a 1.5-hour period. The product was kept at 110°C. for an additional 2 hours following completion of the PO addition.The product, which contained PTB (56%), mono-propoxylated PTB (30%), andheavies (24%), was distilled through an Oldershaw column to givemono-propoxylated PTB (771 g) in 99.8% purity. (B.p. 114° C. @ 40 mm).

METHOD OF APPLICATION OF COATINGS

All coatings were drawn down onto "BONDERITE 1000" steel panels usingthe wire-wound rod necessary to obtain a one-mil dry-film thickness. Thefilms were allowed to dry in air under ambient conditions for seven daysprior to testing by standard ASTM methods.

COMPARATIVE EXAMPLE 1 Water-based Clear Metal Coating-"RHOPLEX WL-91"Resin EB/DB Coalescent

"RHOPLEX WL-91" resin (637.7 parts) was premixed with water (107.9parts) and "PATCOTE 519" defoamer (1.5 parts). Separately, ethyleneglycol n-butyl ether (EB) (79.5 parts) was premixed with diethyleneglycol n-butyl ether (DB) (13.2 parts), di-n-butyl phthalate (13.2parts), and "BYK-301" mar aid (1.33 parts). The glycol ether mixture wasadded to the resin mixture under good mechanical agitation. Mixing wascontinued until a homogeneous blend was obtained.

EXAMPLE 2 Water-based Clear Metal Coating-"RHOPLEX WL-91" ResinPTB/Propoxylated PTB Coalescent

The procedure of Comparative Example 1 was followed except that amixture of propylene glycol tert-butyl ether (PTB) (84.3 parts) andmono-propoxylated PTB (8.3 parts) (prepared as described above) was usedin place of the EB/DB coalescent blend.

                  TABLE 1                                                         ______________________________________                                        Water-borne Coatings from "RHOPLEX WL-91" Resin                               Formulation Example #                                                                         C-1      2                                                    Coalescent      EB/DE    PTB/propoxylated PTB                                 ______________________________________                                        Film formation  yes      yes                                                  Pencil hardness                                                               (ASTM D-3363)                                                                 2 hours         F        H                                                    1 week          H        H                                                    2 weeks         H        2H                                                   MFFT (°C.)                                                                             <-5.0    -4.5                                                 Viscosity (cps)                                                                10 rpm         870      335                                                  100 rpm         265      106                                                  Sward hardness   44       64                                                  (ASTM D2354)                                                                  (30 min)                                                                      ______________________________________                                    

The preparation of water-based clear metal coating compositions based on"RHOPLEX WL-91" resin (Rohm and Haas Company) using either a blend ofEB/DB or PTB/propoxylated PTB is described in Examples 1-2. The physicalproperties of the applied coatings appear in Table 1. Satisfactory filmswith good hardness development and low minimum film-forming temperaturewere formed in each case. The PTB/propoxylated PTB blend had a muchlower viscosity, however, and also produced a film having a superiorSward hardness compared with the EB/DB blend.

COMPARATIVE EXAMPLE 3 Water-Based Clear Gloss Metal Lacquer-"NEOCRYLA-639" Resin EB Coalescent

A premixed blend of water (18.2 parts), EB (92.2 parts), di-n-butylphthalate (15.4 parts), "DC-14" flow aid (4.5 parts), aqueous ammonia(2.6 parts of a 28% solution), and aqueous ammonium benzoate (13.4 partsof a 10% solution) was added slowly with good agitation to "NEOCRYLA-639" resin (681.8 parts). Mixing was continued until a homogeneousblend was obtained. Water (18.2 parts) was added to adjust the viscosityto a value of 45 seconds (#2 Zahn cup).

COMPARATIVE EXAMPLE 4 Water-Based Clear Gloss Metal Lacquer-"NEOCRYLA-639" Resin PTB Coalescent

The procedure of Comparative Example 3 was followed except that PTB(92.2 parts) was used in place of EB.

EXAMPLE 5 Water-Based Clear Gloss Metal Lacquer-"NEOCRYL A-639" ResinPTB/Propoxylated PTB Coalescent Blend

The procedure of Comparative Example 3 was followed except that amixture of PTB (83.9 parts) and propoxylated PTB (8.3 parts) was used inplace of EB.

                  TABLE 2                                                         ______________________________________                                        Water-borne Coatings from "NEOCRYL A-639" RESIN                               Formulation Example #                                                                        C-3    C-4    5                                                Coalescent     EB     PTB    PTB/propoxylated PTB                             ______________________________________                                        Film formation yes    no     yes                                              Pencil hardness                                                               2 hours        3B     --     3B                                               8 hours        B      --     3B                                               1 week         B      --      B                                               Gloss (60°) (ASTM D-523)                                               against grain   95    --     102                                              with grain     111    --     114                                              MFFT (°C.)                                                                            -0.3   --     2.3                                              Viscosity (cps)                                                                10 rpm        172    147    197                                              100 rpm         78     55     70                                              ______________________________________                                    

The preparation of water-based clear gloss metal lacquer compositionsbased on "NEOCRYL A-639" resin (ICI) using EB, PTB, or a blend of PTBand propoxylated PTB is described in Examples 3-5. The physicalproperties of the applied coatings appear in Table 2. Nonviscousformulations and good films were produced with the PTB/propoxylated PTBcoalescent blend. Surprisingly, PTB alone as a coalescent failed to givesatisfactory film formation with this resin, probably due to prematureevaporation of the coalescent from the resin.

                  TABLE 3                                                         ______________________________________                                        Effect of Propoxylated PTB on Minimum                                         Film-Forming Temperature                                                                  Percent Coalescent on Resin Solids                                            required for film formation at 50° F.                                           EB/DB          Propoxylated                              Resin         EB     (86/14)   PTB  PTB                                       ______________________________________                                        "NEOCRYL A-639"                                                                             31     --        69   24                                        "RHOPLEX WL-91"                                                                             --     31        67   27                                        ______________________________________                                    

A key advantage of propoxylated PTB coalescents is their ability toefficiently reduce the minimum film-forming temperature of water-bornecoatings. Surprisingly, we found that very little propoxylated PTBcoalescent was required for satisfactory film formation at 50° F. wheneither "RHOPLEX WL-91" or "NEOCRYL A-639" was used as the resin (Table3). The amount of propoxylated PTB coalescent needed was substantiallyless than the amount of EB or EB/DB blend required, and was far superiorin comparison with PTB.

                  TABLE 4                                                         ______________________________________                                        Effect of Coalescent on pH/Viscosity Profile of                               "RHOPLEX WL-91" Formulation                                                           Viscosity (cps)                                                       pH        EB/DB(86/14)                                                                              Propoxylated PTB                                        ______________________________________                                        7.3       --           59                                                     7.6        270        --                                                      8.5       2250        670                                                     9.0       2100        530                                                     9.5       2400        350                                                     10.3       650        --                                                      10.6      --           79                                                     ______________________________________                                    

Prior art coalescents have an undesirable tendency to swell coatingresins. This swelling tendency, which is normally pH-dependent, resultsin pH-sensitive coating viscosities. PTB alone does not exhibit theswelling problems commonly observed with EB, but PTB alone is often apoor coalescent (see Tables 2 and 3).

Surprisingly, we found that propoxylated PTB coalescents avoid theswelling problems with prior art coalescents while also maintainingsatisfactory film-forming characteristics. Using propoxylated PTB in the"RHOPLEX WL-91" formulation gave coatings having viscosities that weremuch less sensitive to changes in pH compared with the EB/DB coalescent(Table 4). The peak viscosity of the "RHOPLEX WL-91" formulation withthe propoxylated PTB coalescent (670 cps) was 3 or 4 times less thanthat observed with EB/DB (2400 cps). This is an important advantagebecause it gives formulators more control over product viscosity, andallows them to formulate at higher solids (lower VOC) levels whilemaintaining the desired viscosity.

    ______________________________________                                        Trademark          Product of                                                 ______________________________________                                        "RHOPLEX WL-91" RESIN.sup.1                                                                      Rohm and Haas Company                                      "NEOCRYL A-639" RESIN.sup.2                                                                      ICI                                                        "PATCOTE 519" defoamer.sup.3                                                                     Patco Chemical                                             "BYK 301" mar aid.sup.4                                                                          Byk-Chemie                                                 "DC-14" flow aid.sup.5                                                                           Dow Corning                                                "BONDERITE 1000" steel panels                                                                    Parker Chemical Company                                    "ARCOSOLV PTV" glycol ether.sup.6                                                                ARCO Chemical Company                                      ______________________________________                                    

We claim:
 1. A versatile coalescing agent for use in water-bornecoatings that have good film-forming characteristics and rapid hardnessdevelopment upon application, said coalescing agent comprising:(a) fromabout 1 to 10 weight percent of one or more propylene oxide adducts ofpropylene glycol tert-butyl ether having the formula: ##STR6## wherein nis an integer from 2 to 6, and A is an oxypropylene unit; and (b) fromabout 90 to 99 weight percent of an organic solvent selected from thegroup consisting of alcohols, glycols, glycol ethers, glycol etheresters, ketones, esters, aliphatic and aromatic hydrocarbons,halogenated hydrocarbons, nitriles, amides, and carbonates.
 2. Thecomposition of claim 1 wherein n=2.
 3. The composition of claim 1wherein the organic solvent is a glycol ether is selected from the groupconsisting of propylene glycol tert-butyl ether, propylene glycoln-butly ether, propylene glycol methyl ether, propylene glycol ethylether, and propylene glycol phenyl ether.
 4. The composition of claim 1wherein n=2 and the organic solvent is a glycol ether, said glycol etherbeing propylene glycol tert-butyl ether.